In secondary batteries such as lithium secondary batteries, porous separators are used to avoid contact between a positive electrode and a negative electrode. The separator is usually prepared as a self-supporting film which is distinct from the positive electrode and the negative electrode. The separator is sandwiched by the positive electrode and the negative electrode to form a unit structure (electrode cell), and the resulting electrode cell is wound or the cells are stacked, thereby obtaining a battery.
The separator generally used includes a fine porous film of a polyolefin resin film. Such a separator is produced, for example, by extrusion-molding a molten material including a polyolefin resin composition into a sheet, extracting and removing substances other than the polyolefin resin, and drawing the resulting sheet.
The resin film separator is required to have sufficient mechanical strength to resist breakage during battery production, and thus it is difficult to reduce the film thickness by much. Regarding batteries, in particular a type in which a large number of battery cells are stacked or the battery cell is wound, accordingly, the number of the unit cell layers which can be used, per unit volume of the battery is restricted by the thickness of the separator. This leads to a decrease in a battery capacity. In addition, the resin film separator is poor in durability, and when it is used for a secondary battery, a disadvantage occurs in which the separator deteriorates in repeated charging and discharging, thus resulting in a decrease of the cycle characteristics of the battery.